1. Field of the Invention
The present invention relates to a resonant-tunneling heterojunction bipolar transistor (RHBT), having a superlattice structure between an emitter and a base, through which carriers are resonant-tunneled, and a PN junction between the base and a collector.
2. Description of the Related Arts
The resonant-tunneling phenomena has been long known, and recently, due to advances in semiconductor processing technology, such as molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD), practical RHETs have been realized (e.g., "A NEW FUNCTIONAL, RESONANT-TUNNELING HOT ELECTRON TRANSISTOR (RHET)", N. Yokoyama, et al, Japanese Journal of Applied Physics, Vol. 24, No. 11, November, 1985, pp. L853-L854). The RHET includes a superlattice structure between an n-type emitter and an n-type base and a collector barrier between the n-type base and an n-type collector. The RHET has negative differential resistance characteristics, and accordingly, can be used for ternary logic circuits with a high speed, a compact circuit arrangement, and a large margin, etc., such as, tristable gate circuits or a tristate memory cell. However, the RHET suffers from the disadvantages of a poor current gain and the impossibility of operation under a normal temperature, due to the existence of the collector barrier provided to give the tolerance to the reverse voltage. The above disadvantages will be described later in detail with reference to specific examples.
In addition, another type of resonant-tunneling transistor including a superlattice structure in a base, not between an emitter and the base as set forth above, is known (e.g., "Resonant tunneling transistor with quantum well base and high-energy injection: A new negative differential resistance device: F. Capasso, et al, J. Appl. Phys. 53(3), August 1985, PP. 1366-1368). However, in this resonant-tunneling transistor, electrons from the emitter must be ballistically launched at the base and pass into a narrow sub-band in the superlattice structure, and consequently, this transistor is also subjected to a low current gain.